Jenny Hoffman

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Imaging the granular structure of high-T_c superconductivity in underdoped Bi₂Sr₂CaCu₂O_8+x

K.M. Lang,¹ V. Madhavan,¹ J.E. Hoffman,¹ E.W. Hudson^1,2,3 H. Eisaki,^4,§ S. Uchida,⁴ J. C. Davis^1,3
1Department of Physics, University of California, Berkeley, California 94720,, USA
²Department of Physics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139-4301, USA
³Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
^*Department of Superconductivity, University of Tokyo, Yayoi, 2-11-16 Bunkyoku, Tokyo 113-8656, Japan
^§ Present addresses: Department of Applied Physics, Stanford University, Stanford, Califronia 94305-4060, USA

Nature Volume 415 412-416 (24 January 2002).

Abstract

Granular superconductivity occurs when microscopic superconducting grains are separated by non-superconducting regions; Josephson tunnelling between the grains establishes the macroscopic superconducting state. Although crystals of the copper oxide high-transition-temperature (high-T_c) superconductors are not granular in a structural sense, theory suggests that at low levels of hole doping the holes can become concentrated at certain locations resulting in hole-rich superconducting domains. Granular superconductivity arising from tunnelling between such domains would represent a new view of the underdoped copper oxide superconductors. Here we report scanning tunnelling microscope studies of underdoped Bi₂Sr₂CaCu₂O_8+x that reveal an apparent segregation of the electronic structure into superconducting domains that are ~3 nm in size (and local energy gap <50 meV), located in an electronically distinct background. We used scattering resonances at Ni impurity atoms as 'markers' for local superconductivity; no Ni resonances were detected in any region where the local energy gap Delta > 50 ± 2.5 meV. These observations suggest that underdoped Bi₂Sr₂CaCu₂O_8+x is a mixture of two different short-range electronic orders with the long-range characteristics of a granular superconductor.

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For some perspective on this issue you can read the News and Views article about this paper by Jan Zaanen:
"High-temperature superconductivity: Quantum salad dressing"
Nature 415, 569-570 (24 January 2002).

[full text] [pdf]